Polycrystalline silicon films can be vapor deposited by sputtering or chemical deposition on numerous types of substrates including ceramic, but the grain size of the resulting layers is small compared to the dip-coated layer taught herein. Small grain size is not conducive to solar cells for moderate efficiency.
Prior to this invention there was no known method for coating silicon on an inexpensive ceramic substrate, having compatible thermal expansion characteristics, by dipping in molten silicon. Materials such as graphite or carbon are wet by silicon and can be dip-coated, but the large thermal expansion mismatch causes deformity and/or cracking. Ceramic substrates can be chosen and/or synthesized with thermal expansion characteristics similar to those of silicon. Ceramics, however, when dipped into molten silicon, are not wet by the silicon; hence, no coating takes place.
By means of the present invention, it has been discovered that if the ceramic is carbonized prior to being dipped into silicon, a thin film of silicon forms when the substrate is dipped into molten silicon and removed. The carbon layer first reacts with the molten silicon to form a silicon-carbon compound which we presume to be silicon carbide (SiC). As the SiC is formed, it is readily wet by the silicon. When the substrate is withdrawn from the melt, it produces a large grain silicon coating whose thickness can be controlled by the speed with which the substrate is pulled from the melt and/or the temperature of the melt.
More specifically, this invention describes one method for adequately carbonizing the ceramic substrate and it describes temperatures and pull rates which can be used to form large area silicon layers which adhere to the ceramic substrate. Polycrystalline layers formed by pulling a substrate from molten silicon have large individual crystallites and the crystallite size can be much greater than the thickness of the silicon layer.